Known examples of low-loss optical fibers include an optical fiber that includes a core, which is made of silica based glass containing Cl, and a first cladding, which is made of silica based glass containing fluorine so as to have a refractive index lower than the refractive index of the core. Here, the concentration of Cl in the core is about 10 atomic ppm or higher and 20,000 atomic ppm or lower. An example of a more detailed refractive index structure of such optical fibers is known as a depressed cladding structure. In the depressed cladding structure, a cladding includes a first cladding and a second cladding. The first cladding, which surrounds the core, has a refractive index lower than the refractive index of the core. The second cladding, which surrounds the first cladding, has a refractive index lower than the refractive index of the core and higher than the refractive index of the first cladding.
An optical fiber preform for making a fiber having such a depressed cladding structure can be made as described below. A rod made of silica based glass containing Cl, which is to become the core of the optical fiber, is made; and a pipe made of silica based glass containing fluorine, which is to become the first cladding of the optical fiber, is made. The rod is inserted into the pipe, and a glass intermediate is made by heating and integrating the pipe and the rod. A glass layer to become the second cladding of the optical fiber is formed around the glass intermediate, and thereby an optical fiber preform is made. An optical fiber having the depressed cladding structure can be made by drawing the optical fiber preform.
A rod-in-collapse process is an example of a method for forming a glass layer, which is to become a second cladding of an optical fiber, around a glass intermediate. In the rod-in-collapse process, a pipe made of silica based glass containing fluorine, which is to become the second cladding, is made; a glass intermediate is inserted into the pipe; and an optical fiber preform is made by heating and integrating the pipe and the glass intermediate.
VAD and OVD are other examples of a process for forming a glass layer to become a second cladding of an optical fiber around a glass intermediate. In these cases, SiO2 soot is deposited on an outer peripheral surface of the glass intermediate by using VAD or OVD, the SiO2 soot is sintered into transparent glass in a fluorine-containing atmosphere, and thereby an optical fiber preform is made.